The present invention relates to keyed locks, and more particularly to a rotatable cylinder lock employing a sidebar as its primary mechanism for restricting cylinder rotation. The application claims priority of provisional application Ser. No. 62/184,373 filed 25 Jun. 2015.
Sidebars are commonly used in cylinder locks to restrict rotation of the cylinder. These are primarily used in locks that employ tumblers that both slide and rotate, such as Medeco-type locks exemplified by its U.S. Pat. No. 7,779,973 issued 21 Sep. 2010, in typical locks of this type, the sidebar introduces additional degrees of freedom that must be aligned in order for the cylinder to rotate as compared to a conventional sliding tumbler lock. In these cases, rotating the tumblers allows lugs on the sidebar to enter a groove or hole on the tumbler, whereas sliding the tumbler aligns the edge of the tumbler with the shear line. However, there exist some types of lock that use a sidebar as the sole mechanism to limit cylinder rotation. In one such lock of this type, (exemplified by U.S. Pat. No. 3,722,240 issued 27 Mar. 1973) the lugs on the sidebar are only aligned with the holes on tumblers if they are correctly positioned in both elevation and rotation.
Locks of the latter type offer a high degree of security against lock picking techniques. This is not only because they introduce extra degrees of freedom resulting in a larger number of possible key combinations, Rather, it is, to a large extent, because they require each tumbler to be simultaneously aligned along two different axes, or degrees of freedom, before it will exhibit any change in behavior that the lock picker can use to gain useful knowledge of the correct key combination. This contrasts with sliding tumblers, which have only one degree of freedom, therefore guaranteeing that, if they are traversed across their full range of motion, the correct tumbler position will eventually be reached; the question is merely whether the lock picker is skillful enough to detect and interpret the resulting change in behavior. In a tumbler with two degrees of freedom that must be simultaneously aligned, there is no guarantee that any given search pattern will bring the tumbler close enough to correct alignment to produce a useful change in behavior, unless the precision of the lock is also known. Even then, the resulting search pattern will be extremely time consuming to execute State of the art generally is indicated in U.S. Pat. No. 5,018,376 (Lee), U.S. Pat. No. 6,481,255 (Theriault et al), U.S. Pat. No. 7,685,854 (Xu et al); U.S. Pat. No. 7,797,973 (Field et al, Le., the Medeco patent mentioned above); U.S. Pat. No. 8,448,485 (Widen); and U.S. Pat. No. 8,635,022 (Nguyen et al).
Some problems with locks of this type are related to cost and convenience. Using a sidebar increases the part count and especially feature count of the lock, resulting in a lock that is more expensive to manufacture. Also, the keys required by this type of lock are more complicated, making them more difficult to manufacture or to duplicate.
Another issue with locks of this type is that the advantage of having two degrees of freedom per tumbler is reduced if the sidebar pins interface with a hole on a curved surface. This is because, once the center of the sidebar lug is partially over the hole, the pin will be able to depress slightly, since the lug is supported at a point that is not along the shortest path between the lug and the center of the tumbler. This provides information that allows a lock picker to use a relatively loose search pattern to find approximately where the correct tumbler position is, then use a finer search pattern to achieve exact alignment.
Yet another issue is the size of the lugs on the sidebar. This is a result of a design trade-off created by the aforementioned problem of having a curved tumbler surface. If the lugs are too large, the range of tumbler positions in which the tumbler lug will be able to partially depress is large. On the other hand, if the tugs are too small, the lock will be vulnerable to forcing. Such a trade-off prevents the design of a lock that is strong against both lock picking and forcing. For example, the drawings in U.S. Pat. No. 3,722,240 depict tumbler lugs of relatively small size.
Finally, even if the radius of the sidebar lugs is increased, the cross-sectional area of the sidebar lugs is still small compared to the sidebar itself, thus the lock will still not achieve optimal forcing resistance.